[llvm] r366750 - [SimplifyCFG][NFC] Test that we fail to flatten CFG in JPEG "sign" value extend pattern
Roman Lebedev via llvm-commits
llvm-commits at lists.llvm.org
Mon Jul 22 15:09:02 PDT 2019
Author: lebedevri
Date: Mon Jul 22 15:09:02 2019
New Revision: 366750
URL: http://llvm.org/viewvc/llvm-project?rev=366750&view=rev
Log:
[SimplifyCFG][NFC] Test that we fail to flatten CFG in JPEG "sign" value extend pattern
This comes up in JPEG decoding, see e.g.
Figure F.12 – Extending the sign bit of a decoded value in V
of ITU T.81 (JPEG specification).
Added:
llvm/trunk/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll
Added: llvm/trunk/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll?rev=366750&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll (added)
+++ llvm/trunk/test/Transforms/SimplifyCFG/signbit-like-value-extension.ll Mon Jul 22 15:09:02 2019
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; This looks like a normal sign extension, but it's not.
+; This comes up in JPEG decoding. This check is really unpredictable,
+; so it is likely beneficial to avoid the branch.
+
+define i32 @extend_value(i32 %storage, i32 %nbits) {
+; CHECK-LABEL: @extend_value(
+; CHECK-NEXT: bb:
+; CHECK-NEXT: [[SKIPNBITS:%.*]] = sub i32 32, [[NBITS:%.*]]
+; CHECK-NEXT: [[VALUE:%.*]] = lshr i32 [[STORAGE:%.*]], [[SKIPNBITS]]
+; CHECK-NEXT: [[SHOULDEXTEND:%.*]] = icmp sgt i32 [[STORAGE]], -1
+; CHECK-NEXT: br i1 [[SHOULDEXTEND]], label [[EXTEND:%.*]], label [[END:%.*]]
+; CHECK: extend:
+; CHECK-NEXT: [[HIGHBITMASK:%.*]] = shl nsw i32 -1, [[NBITS]]
+; CHECK-NEXT: [[HIGHBITMASKPLUSONE:%.*]] = add nsw i32 [[HIGHBITMASK]], 1
+; CHECK-NEXT: [[EXTENDED:%.*]] = add i32 [[HIGHBITMASKPLUSONE]], [[VALUE]]
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ [[EXTENDED]], [[EXTEND]] ], [ [[VALUE]], [[BB:%.*]] ]
+; CHECK-NEXT: ret i32 [[RES]]
+;
+bb:
+ %skipnbits = sub i32 32, %nbits
+ %value = lshr i32 %storage, %skipnbits
+ %shouldextend = icmp sgt i32 %storage, -1
+ br i1 %shouldextend, label %extend, label %end
+
+extend: ; preds = %bb
+ %highbitmask = shl nsw i32 -1, %nbits
+ %highbitmaskplusone = add nsw i32 %highbitmask, 1
+ %extended = add i32 %highbitmaskplusone, %value
+ br label %end
+
+end: ; preds = %extend, %bb
+ %res = phi i32 [ %extended, %extend ], [ %value, %bb ]
+ ret i32 %res
+}
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